The Notch pathway is involved in cell fate determination and differentiation, and signaling through the Notch pathway receptors is an evolutionarily conserved mechanism for cell-cell interaction. Notch proteins, of which four have been identified in humans (Notch 1, Notch 2, Notch 3 and Notch 4), are a family of closely related transmembrane receptors. Notch 4 is expressed specifically in endothelial cells (Shiratoshi, Y., Genes Cells 2:213-224, 1997; Uyttendaele, H., Development 122:2251-2259, 1996) and may play an important role in angiogenesis. When Notch is activated by a ligand, its intracellular domain is proteolytically cleaved and transported to the nucleus, along with CSL (CBF-1/Su(H)/Lag-1/RBP-JK) transcription factor to activate transcription of downstream effectors. The resulting effector can repress the transcriptional activity of other genes encoding transcription factors for entry into terminal differentiation. The ligands that interact with the extracellular portion of Notch include Delta, Serrate, and Jagged; the ligands also are transmembrane proteins.
Adjacent cells of identical lineage can follow separate pathways of differentiation as a result of the Notch pathway. Sample pathways of differentiation include axis formation, cartilage formation, and somite formation. Through a process of lateral inhibition, one cell can suppress the neighboring cells from following the same path of differentiation. In one model, a Notch receptor is expressed on the cell surface of a “suppressed” cell, and interacts with a Notch ligand located on the cell surface of a dominating cell. After ligand interaction with a Notch receptor, the intracellular domain of the Notch receptor is cleaved and transported to the nucleus, where it forms a complex and affects gene transcription. (Lendahl, U., BioEssays 20:103-107, 1998.)
The ligand itself plays an important role in determining the fate of cells in the vicinity of Notch-expressing cells. In Drosophila, ligands including Delta and Serrate have been identified and studied. The corresponding genes in mammals include Dll-1 (Delta-1) and Dll-3 (Delta-3), as well as Jag-1 and Jag-2. Mutations and decreased expression of Delta are related to p henotypic changes, and a translocation in the human Notch 1 locus (TAN-1) has been found in T-cell acute lymphoblastic leukemia/lymphoma (Ellisen et al., Cell 66:649-661, 1991).
Mutations in the human Jagged 1 gene are associated with Alagille syndrome, which involves abnormal development of liver, heart, skeleton, eye, and face. Alagille patients also exhibit valvular and arterial stenonis and high incidence of intracranial hemorrhage. Four separate mutations, all frameshifts, have been identified in patients with the syndrome. (Li, L. et al., Nature Genetics 16:243-251, 1997; Oda, T. et al., Nature Genetics 16:235-242, 1997.) The mutations are likely to interfere with the ability of Jagged 1 to interact with Notch, thereby affecting the differentiation of cells whose fate would otherwise be determined by interaction of Notch with functional Jagged 1. Mice rendered genetically deficient for Jagged 1 exhibit defects in vascular development (Xue et al., Hum. Mol. Gen. 8:723, 1999). These results are consistent with the hypothesis that Jagged 1 is also involved in vascular development and integrity.
Mutations in Notch 3 are related to a syndrome known as CADASIL, for cerebral autosomal arteriopathy with subcortical infarction and leukoencephalopathy. (Jontel, A. et al., Lancet 350:1511-1515, 1997.) Missense mutations in the extracellular domain were found in 45 out of 50 CADASIL patients in one reported study. (Salloway, S. et al, J. Geriatr. Psychiatry Neurol. 11:71-77, 1998.) CADASIL patients exhibit recurrent ischemic stroke and severe vascular smooth muscle cell defect. Thus, mutations in the Notch gene itself can affect vascular integrity in adults.
In view of the importance of this signaling pathway and its role in human cell differentiation and disease, there is a need in the art for identification of genes involved in the pathway, and for methods and therapeutic agents for intervening in diseases and conditions related to defects in the Notch pathway.